1. Field of the Invention
The present invention relates to an automatic gain control (AGC) circuit and method, and more particularly to an automatic gain control circuit and method capable of controlling a margin voltage.
2. Description of the Related Art
In the wireless communication field, an on-off keying radio-frequency (RF) receiver of a super-regenerative architecture has better sensitivity and better noise prevention efficiency/cost ratio. Thus, it has been widely applied in apparatuses such as remote-control toy cars or other remote-control electrical devices. In the circuit structure, an automatic gain control circuit is required to control the start-up time of an oscillator to maintain the best performance.
FIG. 1 is a drawing showing signal modulations of an on-off keying RF receiver. Referring to FIG. 1, when the input data Din is 1, the output signal Sout is a carrier-modulated high-level signal. If the input data Din is 0, the level of the output signal Sout is also 0.
FIG. 2 is a circuit block diagram showing a super-regenerative RF receiver. FIG. 3 is a schematic drawing showing signal relationships of a super-regenerative RF receiver. Referring to FIGS. 2 and 3, the saw-wave generator 220 generates the saw-wave output current lq based on the quench control signal qnch. The oscillator 210 receives the saw-wave output current lq and the RF signal Srf to generate the oscillation output Sosc. When the received RF modulated signal Srf is high, it provides higher voltage to trigger the oscillator 210, so that the oscillator 210 generates an earlier start-up time and larger output envelope of the oscillator Sosc, as shown in FIG.3. Similarly, when the received RF modulated signal Srf is low, the oscillator 210 provides a relative late start-up time and smaller output envelope of the oscillator Sosc. Accordingly, the received data can be regenerated and reverted by the post-level circuit.
In order to stabilize the oscillation output Sosc of the oscillator 210, the super-regenerative RF receiver comprises an automatic gain control circuit 230 as shown in FIG. 2. By controlling the value of the saw-wave output current lq output from the saw-wave generator 220, the start-up time of the oscillation output of the oscillator 210 is adjusted.
A conventional automatic gain control circuit is disclosed in “A low-power CMOS super-regenerative receiver at 1 GHz”, p. 440, vol. 36, IEEE JSSC. In the conventional circuit, the functionality of a gm-C low pass filter is to detect an energy level of the oscillation envelope. Nevertheless, the gm-C low pass filter requires a lot of large size gm-C cell to realize resistors and inductors, so it needs a lot of layout size. Also, the response time of the conventional automatic gain control circuit is slow because it needs some several bits of a data frame to obtain the signal level of the input. Since it is always in the state of tracing the operational status of signals, a great number of noises are thus generated.
In order to overcome the issue described above, the U.S. Pat. No. 6,864,756 discloses an automatic gain control circuit to stabilize a start-up time of an oscillator. In the U.S. Pat. No. 6,864,756, the automatic gain control circuit directly processes the output of the oscillator but the issue that the output of the oscillator has many noises is left unanswered. In addition, the automatic gain control circuit of the 6,864,756 patent needs to receive an external RF signal Srf, while automatic-gain controlling the oscillator. That also enhances the chance of noise interference.